pub fn dp_rec_mc(amount: u32) -> u32 {
    let v = vec![1,2,5,10,20,30,50,100];
    let mut nums = 0;
    let mut left = amount;
    v.iter().rev().for_each( |x| {
        nums += left / x;
        left = left % x;
    }
    );
    nums
}
